This invention relates to controls for pneumatically powered linear actuators which include a driven elongated threaded member or lead screw that positively positions the linear actuator.
The characteristics of a pneumatic cylinder include a capability for producing great forces through the use of a readily available source of power utilizing simple inexpensive constructions. Pneumatic cylinders have a notable disadvantage in that, once in motion, it is very difficult to stop the piston rod or other operating mechanism in a precisely desired position. Electric cylinders or linear actuated devices powered by an electric motor as may be operated through the use of a screw threaded drive are relatively simple in design offering precise positioning in fast or slow motion so as to offer both position and velocity control. Such systems utilize a clean readily available power source with little or no noise or leakage problems. Since the electrically driven actuators utilize a common source of power they may interface with controllers, computers and the like. Very small power to weight ratios however, make them inefficient from the standpoint of space considerations. Moreover, loads may back drive the cylinder.
Servo controlled air cylinders provide power to weight ratios conducive to space efficiency. A feed back system may be provided for positioning each cylinder and integrated with a system of valves and feedback devices operating through a dedicated controller. Such cylinders may be infinitely positionable or positioned in discrete stroke increments. Closed loop hydraulic systems offer comparable results to electric cylinders but such cannot be utilized in pneumatic systems due to compressibility of the media. Suitable velocity control characteristics are not provided by such complicated systems.
Accordingly, it is desired to obtain positive positioning and velocity control in a pneumatically powered linear actuator by a simplified operating mechanism wherein an electric motor controls the operation of a valve for distributing pressurized air for powering the linear actuator and for fixing the linear actuator in a desired position.